unmount the filesystem and check the permissions/ownership of the underlying directory/mount point and correct as required to ensure your ZIMBRA app ID can rwx the mountpoint. Then mount up the filesystem and check permissions/ownership again and correct as required.

Then add the hdisk to the required volume group and extend de filesystem.
Only then any application (not only this ZIMBRA) will see any disk space
change. If you need to restart a mail server (note that I know nothing at
all about ZIMBRA) I think that you need a new mail server.